This sounds like a combination of a DTMF relay problem and codec mismatch.

Make sure that the third phone has a transcoder available, and check the dial peers you're matching for calls with 'debug voip dialpeer' to make sure you're matching the right dial peer for each call leg.

This sounds like a combination of a DTMF relay problem and codec mismatch.

Make sure that the third phone has a transcoder available, and check the dial peers you're matching for calls with 'debug voip dialpeer' to make sure you're matching the right dial peer for each call leg.